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Blastocystis hominis

[Blastocystis hominis]

Causal Agents

The taxonomic classification of Blastocystis hominis is mired in controversy. It has been previously considered as yeasts, fungi, or ameboid, flagellated, or sporozoan protozoa. Recently, however, based on molecular studies, especially dealing with the sequence information on the complete SSUrRNA gene, B. hominis has been placed within an informal group, the stramenopiles (Silberman et al. 1996). Stramenopiles are defined, based on molecular phylogenies, as a heterogeneous evolutionary assemblage of unicellular and multicellular protists including brown algae, diatoms, chrysophytes, water molds, slime nets, etc. (Patterson, 1994). Cavalier-Smith (1998) considers stramenopiles to be identical to his infrakingdom Heterokonta under the kingdom Chromista. Therefore, according to Cavalier-Smith, B. hominis is a heterokontid chromista.

References:

 

  • Silberman JD, Sogin ML, Leipe DD, Clark CG. Human parasite finds taxonomic home. Nature 1996;380:398.
  • Patterson DJ. Protozoa, evolution and systematics. In: Housmann K, Hulsmann N, editors. Progress in Protozoology. Stuttgart: Fischer; 1994. p. 1-14.
  • Cavalier-Smith T. A revised six-kingdom system of life. Biol Rev Camb Philos Soc 1998;73:203-266.

 


Life Cycle

lifecycle

Blastocystis hominis stages were reproduced from Singh M, Suresh K, Ho LC, Ng GC, Yap EH. Elucidation of the life cycle of the intestinal protozoan Blastocystis hominis. Parasitol Res 1995;81:449. Copyright held by Springer-Verlag and reproduced here by permission of Springer-Verlag and M. Singh.

Knowledge of the life cycle and transmission is still under investigation, therefore this is a proposed life cycle for B. hominis. The classic form found in human stools is the cyst, which varies tremendously in size from 6 to 40 µm The Number 1. The thick-walled cyst present in the stools The Number 1 is believed to be responsible for external transmission, possibly by the fecal-oral route through ingestion of contaminated water or food The Number 2. The cysts infect epithelial cells of the digestive tract and multiply asexually (The Number 3, The Number 4 ). Vacuolar forms of the parasite give origin to multi vacuolar The Number 5a and ameboid forms The Number 5b. The multi-vacuolar develops into a pre-cyst The Number 6a that gives origin to a thin-walled cyst The Number 7a, thought to be responsible for autoinfection. The ameboid form gives origin to a pre-cyst The Number 6b, which develops into thick-walled cyst by schizogony The Number 7b. The thick-walled cyst is excreted in feces The Number 1.

Geographic Distribution

Worldwide.

Clinical Presentation

Whether Blastocystis hominis can cause symptomatic infection in humans is a point of active debate. This is because of the common occurrence of the organism in both asymptomatic and symptomatic persons. Those who believe symptoms could be related to infection with this parasite have described a spectrum of illness including watery diarrhea, abdominal pain, perianal pruritus, and excessive flatulence.

Blastocystis hominis cyst-like forms in wet mounts.

Blastocystis hominis appear as spherical to oval cyst-like structures. They vary widely in size (5 to 30 µm; usual range 8 to 10 µm), and typically consist of a central body, or "vacuole," surrounded by a thin rim of cytoplasm containing up to six nuclei. The vacuoles stain variably from red to blue in trichrome stain, which is preferred to wet mounts as the organisms can be overlooked as debris.

Figure A

Figure A: B. hominis cyst-like forms in a wet mount, unstained.

Figure B

Figure B: B. hominis cyst-like form in a wet mount, unstained.

B. hominis cyst-like forms in wet mounts under differential interference contrast (DIC) microscopy.

Figure A

Figure A: B. hominis cyst-like forms in wet mounts under differential interference contrast (DIC) microscopy.

Figure B

Figure B: B. hominis cyst-like forms in wet mounts under differential interference contrast (DIC) microscopy.

B. hominis cyst-like forms in wet mounts stained with iodine.

Figure A

Figure A: B. hominis cyst-like forms in wet mounts stained in iodine.

Figure B

Figure B: B. hominis cyst-like forms in wet mounts stained in iodine.

B. hominis cyst-like forms stained with trichrome

Figure A

Figure A: B. hominis cyst-like forms stained with trichrome. The nuclei in the peripheral cytoplasmic rim are visible, staining purple.

Figure B

Figure B: B. hominis cyst-like forms stained with trichrome. The nuclei in the peripheral cytoplasmic rim are visible, staining purple.

Figure C

Figure C: Blastocystis hominis cyst-like forms stained with trichrome.

Figure D

Figure D: Blastocystis hominis cyst-like forms stained with trichrome.

Figure E

Figure E: B. hominis cyst-like forms stained with trichrome.

Figure F

Figure F: B. hominis cyst-like forms undergoing binary fission; stained with trichrome.

Figure G

Figure G: B. hominis cyst-like forms stained with trichrome.

Laboratory Diagnosis

Diagnosis is based on finding the cyst-like stage in feces. Permanently stained smears are preferred over wet mount preparations because fecal debris may be mistaken for the organisms in the latter. Do not wash specimens in water (e.g., during concentration procedures) as this will lyse the organisms, resulting in false negatives.

Morphology

More on: Morphologic comparison with other intestinal parasites

Bench Aids

A thumbnail for the linked document.Laboratory diagnosis of Blastocystis hominis infection -- Blastocystis hominis Adobe PDF file [PDF, 83 KB, 1 Page]

Treatment Information

The clinical significance of Blastocystis spp. is controversial.

Treatment with metronidazole* at various doses has been reported, for example (adults):

  • 250 mg to 750 mg metronidazole* orally 3 times daily for 10 days
  • 1500 mg metronidazole* orally once daily for 10 days

Note: Lack of response to metronidazole has been noted in some areas (Yakoob et al., Br J Biomed Sci 2004;61:75).

Treatment with trimethoprim (TMP)*/sulfamethoxazole (SMX)* at various doses has been reported, for example (adults):

  • 6 mg/kg TMP*, 30 mg/kg SMX* once daily for 7 days
  • 320mg TMP*, 1600 mg SMX* once daily for 7 days
  • 160 mg TMP*, 800 mg SMX* twice daily for 7 days

Treatment with nitazoxanide* has been shown to be effective in clearing organisms and improving symptoms at the following doses:

  • Adults, 500 mg nitazoxanide* orally twice daily for 3 days.
  • Children, 200 mg nitazoxanide* orally twice daily for 3 days in patients aged 4–11 years, and 100 mg nitazoxanide* orally twice daily for 3 days in patients aged 1–3 years.

Tinidazole*, paromomycin*, iodoquinol*, and ketoconazole* have also been used for clearing Blastocystis, as presented in case reports or small series (see references).

*Not FDA-approved for this indication.

Metronidazole

Metronidazole is available for human use in the United States.

Metronidazole is in pregnancy category B. Data on the use of metronidazole in pregnant women are conflicting. The available evidence suggests use during pregnancy has a low risk of congenital anomalies. Metronidazole may be used during pregnancy in those patients who will clearly benefit from the drug, although its use in the first trimester is generally not advised.

Pregnancy Category B: Either animal-reproduction studies have not demonstrated a fetal risk but there are no controlled studies in pregnant women or animal-reproduction studies have shown an adverse effect (other than a decrease in fertility) that was not confirmed in controlled studies in women in the first trimester (and there is no evidence of a risk in later trimesters).

Metronidazole is excreted in breast milk. The American Academy of Pediatrics classifies metronidazole as a drug for which the effect on nursing infants is unknown but may be of concern. The World Health Organization (WHO) advises to avoid metronidazole treatment in lactating women. Metronidazole should be used during lactation only if the potential benefit of therapy to the mother justifies the potential risk to the infant.

The safety of metronidazole in children has not been established. Metronidazole is listed as an antiamebic and antigiardiasis medicine on the WHO Model List of Essential Medicines for Children, intended for the use of children up to 12 years of age.

Trimethoprim–sulfamethoxazole

Trimethoprim–sulfamethoxazole (TMP–SMX) is available for human use in the United States.

Trimethoprim–sulfamethoxazole (TMP–SMX) is in pregnancy category C. TMP–SMX should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. TMP-SMX should be avoided near-term because of the potential for hyperbilirubinemia and kernicterus in the newborn.

Pregnancy Category C: Either studies in animals have revealed adverse effects on the fetus (teratogenic or embryocidal, or other) and there are no controlled studies in women or studies in women and animals are not available. Drugs should be given only if the potential benefit justifies the potential risk to the fetus.

Trimethoprim–sulfamethoxazole (TMP–SMX) is excreted in breast milk. TMP–SMX generally is compatible with breastfeeding of healthy, full-term infants after the newborn period. However, TMP-SMX generally should be avoided by women when nursing infants who are premature, jaundiced, ill, or stressed, or who have glucose-6-phosphate dehydrogenase deficiency.

The safety of trimethoprim–sulfamethoxazole (TMP–SMX) in children has not been systematically evaluated. Use in children less than 2 months of age generally is not recommended.

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DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/parasites/.

  • Page last reviewed: May 3, 2016
  • Page last updated: May 3, 2016
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